The present invention generally relates to zeolites and materials having zeolite-type frameworks. The invention particularly relates to processes for producing silicates having zeolite-type frameworks via liquid-phase reflux procedures.
Zeolites are crystalline aluminosilicates and members of a family of microporous solids known as molecular sieves, which have the ability to selectively sort molecules based primarily on a size exclusion process. This is due to a relatively regular pore structure of molecular dimensions. As such, zeolites are commonly used as commercial adsorbents and catalysts, especially in the chemical and petrochemical industries.
Zeolites are composed of a three-dimensional framework generally comprising connected tetrahedral [AlO4/2]− and [SiO4/2] subunits, with each subunit linked in a fourfold coordination by sharing its oxygen atoms with other such tetrahedra. Each [AlO4/2]− in the framework carries a net negative charge, which is balanced by an extra-framework cation (for example, H+, K+, or Na+). Over the course of synthesis of zeolites, the primary tetrahedral subunits are assembled into secondary building units, which are simple polyhedra such as cubes, hexagonal prisms, or cuboctahedra. The final framework structure then comprises a repeating arrangement of these secondary building units. Because there are many possibilities for the secondary building units to assemble in three dimensions, there exists a large number of crystallographically unique structures (referred to as frameworks or framework structures).
Various processes exist to replace heteroatoms that already exist in a zeolite framework with other heteroatoms in order to produce silicate molecular sieves having metal-containing zeolite-type frameworks (referred hereinafter as zeotype materials) with a variety of properties. The term heteroatom refers to elements, such as tin, titanium, zirconium, hafnium, gallium, iron, boron, germanium, beryllium, vanadium, chromium, etc., which are incorporated into the zeolite framework by partial isomorphous substitution of the typical framework heteroatoms (for example, silicon, aluminum, and phosphorus). Generally, such substitutions are intended to adjust properties of the material (e.g., Lewis acidity) for a certain application. Zeotype materials have been used for a variety of applications for chemical synthesis, pharmaceutical synthesis, and biomass conversion, including sugar isomerizations, oxidations of ketones and aldehydes, epoxidation of olefins, etc.
Current synthesis processes to prepare zeotype materials are limited in the amount of heteroatoms that can be incorporated (chemically-bonded) into the zeolite-type framework (and not just physically deposited onto the oxide surface). In addition, current processes do not allow for control over the coordination of the heteroatoms that are bonded into the framework. For example, in the case of incorporating Sn atoms into the framework to produce a stannosilicate, current applications are unable to control whether the Sn atoms form three bonds (open configuration) or four bonds (closed configuration) to the silica lattice of the framework. In many catalytic applications, only the Sn atoms that are in an open configuration are able to catalyze the reaction involved.
In view of the above, it can be appreciated that there are certain problems, shortcomings or disadvantages associated with the prior art, and that it would be desirable if a process were available for producing zeotype materials with increased amounts of heteroatoms incorporated into their zeolite-type frameworks and improved control over the configuration of heteroatom sites formed therein.